3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard with e.g. increased capacity and higher data rates towards the fourth generation of mobile telecommunication networks. Hence, the LTE specifications provide downlink peak rates up to 300 Mbps, an uplink of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both FDD (Frequency Division Duplex) and TDD (Time Division Duplex).
LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as illustrated in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control is illustrated in FIG. 3.
LTE uses hybrid-ARQ, where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NAK). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
Uplink control signaling from the terminal to the base station consists of hybrid-ARQ acknowledgements for received downlink data; terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling; scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions.
The uplink control data is either multiplexed with granted uplink user data transmissions, or transmitted via an uplink control channel if the UE has not received a grant for uplink scheduled data transmission.
A fundamental requirement for any cellular system is the possibility for the UE to request a connection setup, commonly referred to as random access. Typically, the random access carried out via a contention based random access channel (RACH). In LTE, the RACH is used to achieve uplink time synchronization in different scenarios, where initial access to the cellular system represents one scenario. (Uplink time synchronization is vital for obtaining orthogonal transmissions and is an LTE system requirement for granting any uplink transmission of data.)
The LTE random-access procedure consists of four steps in which the RACH is used in the first step, as illustrated in FIG. 4, for the transmission of a random access preamble. The preamble transmission indicates to the base station the presence of a random access attempt and allows the base station to estimate the propagation delay between the eNodeB and the UE. In the second step, the eNodeB responds by sending a message on the downlink shared channel (DL-SCH) that includes e.g. required uplink timing adjustments and a grant for uplink scheduled transmission of higher layer messages on the uplink shared channel (UL-SCH). In the third step, the UE transmits its random access message, via UL-SCH, which could e.g. be a connection request or an uplink scheduling request if the UE is already connected. This message also includes the identity of the UE. In the fourth step, the eNodeB transmits the contention resolution message containing the UE identity received in the third step.
The time-frequency resource upon which the random access preamble is transmitted is known as the physical RACH (PRACH). Transmission of PRACH is only possible in certain subframes, which are known to the UE through broadcasted system information. The bandwidth of the PRACH transmission is always 6 resource blocks whereas its time duration depends on configured preamble format. FIG. 5 illustrates a PRACH configuration with one PRACH allocation per radio frame and with 1 ms PRACH duration.
The power control used for the transmission of a random access preamble on the PRACH is based on an open loop procedure, i.e. there is no feedback from the eNodeB. Typically the UE bases its initial PRACH power settings on estimated downlink pathloss and the eNodeB preamble received target power available to the UE as part of the broadcasted system information.
Since the random access preamble transmission is a non-scheduled transmission, it is not possible for the eNodeB to employ a closed loop correction to correct for measurement errors in the open loop estimate. Instead, a power ramping approach is used where the UE will increase its transmission power (or rather its received target power) between transmission attempts of the random access preamble. This ensures that even a UE with a too low initial transmission power, due to e.g. error in the pathloss estimate, after a number of preamble transmission attempts will have increased its power sufficiently to be able to be detected by the eNodeB. For example, after N random access attempts, the total ramp-up of the transmission power isΔPrampup=(N−1)*Δramp step where Δramp step is the power ramping step size between each transmission attempt. It is desired to keep the number of random access attempts N to a reasonable low number in order to avoid high probability of random access collisions with other terminals as well as to avoid large access delays.
In order to increase the downlink and uplink peak-data rates in LTE systems, it has been decided for LTE Release 10 to increase the maximum transmission bandwidth from 20 MHz up to 100 MHz. This bandwidth expansion is achieved by aggregating multiple component carriers, where each component carrier has a maximum bandwidth of 20 MHz. Multiple downlink, or uplink, component carries can either be adjacent or non-adjacent. Carrier aggregation thus allows for simultaneous transmissions/receptions on different non-contiguous spectrum fragments.